I have an Ole C gaita. Recently the B (concert A) always plays quite flat. Is anyone familiar with a changing chanter and how to fix it? I've tried changing reeds (I have a few extra). Sometimes it seems doing that helps for about 5 minutes, but then it starts playing flat again. Sometimes the replaced reed doesn't do a thing to it.

Note, I can get it to play an in tune B by putting extra pressure on the bag, but that affects both the sound and the pitch of the drones.

If all the other notes are in tune, that means only one thing. The fingerhole is too narrow.
First have a very good look, to see if anything is obstructing the fingerhole. It can be something inside the bore, or in the fingerhole itself. In any case, clean the bore out, it won't hurt. (To do that, get a wire or a really long bamboo stick, fold a bit of a material around one end, and sort of polish the bore. Don't let the metal of the wire scratch the bore. (hence the bamboo idea.)) Also look into the fingerholes, and remove any fluff, lint, etc. There might be tuning wax there, do not touch that.
If all of this doesn't make any difference, you'll have to enlarge the fingerhole. To do that, roll up very tightly a strip of fine-ish sandpaper, say P120-P150 (that's the fineness grade) , just thin enough to pass into the hole, then file away. A tiny bit, like 2-3 strokes in and out, at one time. Then check the tuning. It certainly won't be enough, so repeat. Until it is in tune. Do not get impatient, as narrowing a fingerhole is a bit more complicated than enlarging it. Take your time. Really.